I've been to both, and 5 days at Disneyland Paris would be perfect at that age, we did that length of stay last year with DD then 5, and the princess meal, it was a wonderful trip and she talks about it constantly. The park is absolutely beautiful.
If Florida is a one off, I would definitely leave it a few years. Yes, they would love it now, but they will love it even more at 9 or 10, or any age beyond.
I loved Florida when I went with my older DC, and will definitely go back, it's amazing, but I don't understand the real aggression you often hear about DLP from Florida goers, they really put it down, and then usually they've never been anyway. It's as though they have to justify the money they are spending by telling everyone within earshot that DLP is a load of crap. Not that they've been, of course, but it probably is. Yeah.
I can honestly say that DLP is utterly magical, it's not on the huge scale of Florida, but that's fine. Florida can come later.